The present invention relates to a broad class of processible stable polymers which are applied to the creation of a high density erasable data storage member intended for use in information storage and processing applications. The polymers in use for such applications are of the polyaniline class. Polyaniline is a family of polymers whose electronic and optical properties are manipulated through a control which directs variations of the number of electrons and protons on the polyaniline polymer chain.
This invention is related to the invention disclosed in co-pending application Ser. No. 193,964, which was filed on May 13, 1988. That application discusses how the chemical and physical properties of the polyaniline class of polymers are modified by chemical and electrical methods such as protonation and oxidative doping. As such, polyaniline and its derivatives are successfully applied to the absorption of electromagnetic radiation and the modulation of electromagnetic beams. The technologies discussing the chemical and electrical methods for modifying the chemical and physical properties of polyaniline are incorporated by reference herein as background relating to the present invention.
Polyaniline is a family of polymers that has been under intensive study recently because the electronic and optical properties of the polymers can be modified through variations of either the number of protons, the number of electrons, or both. The polyaniline polymer can occur in several general forms including the so-called reduced form (leucoemeraldine base), possessing the general formula ##STR1## the partially oxidized so-called emeraldine base form, of the general formula ##STR2## and the fully oxidized so-called pernigraniline form, of the general formula ##STR3##
In practice, polyaniline generally exists as a mixture of the several forms with a general formula (I) of ##STR4##
When 0&lt;y&lt;1 the polyaniline polymers are referred to as poly(paraphenyleneamineimines) in which the oxidation state of the polymer continuously increases with decreasing value of y. The fully reduced poly(paraphenyleneamine) is referred to as leucoemeraldine, having the repeating units indicated above corresponding to value of y=1. The fully oxidized poly(paraphenyleneimine) is referred to as pernigraniline, of repeat unit shown above corresponds to a value of y=0. The partly oxidized poly(paraphenyleneimine) with y in the range of greater than or equal to 0.35 and less than or equal to 0.65 is termed emeraldine, though the name emeraldine is often focused on y equal to or approximately 0.5 composition. Thus, the terms "leucoemeraldine", "emeraldine" and "pernigraniline" refer to different oxidation states of polyaniline. Each oxidation state can exist in the form of its base or in its protonated form (salt) by treatment of the base with an acid.
The use of the terms "protonated" and "partially protonated" herein includes, but is not limited to, the addition of hydrogen ions to the polymer by, for example, a protonic acid, such as mineral and/or organic acids. The use of the terms "protonated" and "partially protonated" herein also includes pseudoprotonation, wherein there is introduced into the polymer a cation such as, but not limited to, a metal ion, M.sup.+. For example, "50%" protonation of emeraldine leads formally to a composition of the formula ##STR5## which may be rewritten as ##STR6##
Formally, the degree of protonation may vary from a ratio of [H.sup.+ ]/[--N.dbd.]=0 to a ratio of [H.sup.+ ]/[--N.dbd.]=1. Protonation or partial protonation at the amine (--NH--) sites may also occur.
The electrical and optical properties of the polyaniline polymers vary with the different oxidation states and the different forms. For example, the leucoemeraldine base, emeraldine base and pernigraniline base forms of the polymer are electrically insulating while the emeraldine salt (protonated) form of the polymer is conductive. Protonation of emeraldine base by aqueous HCl (1M HCl) to produce the corresponding salt brings about an increase in electrical conductivity of approximately 10.sup.10 ; deprotonation occurs reversibly in aqueous base or upon exposure to vapor of, for example, ammonia. The emeraldine salt form can also be achieved by electrochemical oxidation of the leucoemeraldine base polymer or electrochemical reduction of the pernigraniline base polymer in the presence of an electrolyte of the appropriate pH. The rate of the electrochemical reversibility is very rapid; solid polyaniline can be switched between conducting, protonated and nonconducting states at a rate of approximately 10.sup.5 Hz for electrolyte in solution and even faster with solid electrolytes. [E. Paul, et al., J. Phys. Chem., 89, 1441-1447 (1985)]. The rate of electrochemical reversibility is also controlled by the thickness of the film, thin films exhibiting a faster rate than thick films. Polyaniline can then be switched from insulating to conducting form as a function of protonation level (controlled by ion insertion) and oxidation state (controlled by electrochemical potential). Thus, polyaniline can be turned "on" by either a negative or a positive shift of the electrochemical potential, because polyaniline films are essentially insulating at sufficiently negative (approximately 0.00 V vs. SCE) or positive (+0.7 V vs. SCE) electrochemical potentials. Polyaniline can also then be turned "off" by an opposite shift of the electrochemical potential.
Polymers have also recently been under investigation because of the ability to alter their optical properties as the result of exposure to optical excitation. The photoinduced absorption spectrum of polyaniline differs substantially from other polymers such as polyacetylene, polythiophene, polypyrrole and polydiacetylene in several important aspects. First, polyaniline is not charge conjugation symmetric; that is, the Fermi level and band gap are not formed in the center of the .pi. band, so that the valence and conduction bands are very asymmetric. [S. Stafstrom, J. L. Bredas, A. J. Epstein, H. S. Woo, D. B. Tanner, W. S. Huang, and A. G. MacDiarmid, Phys. Rev. Lett. 59, 1464 (1987)]. Consequently, the energy level positions of doping induced and photoinduced excitations differ from those in charge-conjugation-symmetric polymers such as polyacetylene and polythiophene. Second, both carbon rings and nitrogen atoms are within the conjugation path forming a generalized "A-B" polymer, unlike polypyrrole and polythiophene, whose heteroatoms do not contribute significantly to .pi. band formation. [M. J. Rice and E. J. Mele, Phys. Rev. Lett 49, 1455 (1982); J. L. Bredas, B. Themans, J. G. Fripiat, J. M. Andre and R. R. Chance, Phys. Rev. B29, 6761 (1984)]. Third, the emeraldine base form of polyaniline can be converted from an insulating to a metallic state if protons are added to the --N.dbd. state sites while the number of electrons on the chain is held constant. [J. C. Chaing and A. G. Mac Diarmid, Synth. Met. 13, 193 (1986)]. For example, exposure of emeraldine base to a protonic acid such as HCl causes a transformation to the emeraldine salt form of polyaniline. The emeraldine salt form of polyaniline exhibits metallic properties which are due to the formation of a polaron lattice in the material. [J. M. Ginder, A. F. Richter, A. G. MacDiarmid, and A. J. Epstein, Solid State Commun., 63, 97 (1987); A. J. Epstein, J. M. Finder, F. Zuo, R. W. Bigelow. H. S. Woo, D. B. Tanner, A. F. Richter, W. S. Huang and A. G. MacDiarmid, Synth. Met. 18, 303 (1987); H. Y. Choi, and E. J. Mele, Phys. Rev. Lett. 59, 2188 (1987)].
These distinct photoinduced properties of the polyaniline class of polymers provides a unique opportunity for the application of such polymers to technologies outside of the previously accepted variety of applications. One such unique application is the erasable optical information storage technology of this invention.